The toasting process breaks down starch and other polymers, called lignins and tannins, in the oak. Under the charred layer of the barrel is a 'red line,' a layer where the oak was toasted during the charring process of making the barrel. These chemicals bind to the charred layer and do not release, kind of like a water filter.

The charred wood acts like a filter and strains out some of the chemicals before the distillate seeps into the wood. During the summer, when the temperature is hot, the distillate can pass through the inner charred layer of the barrel. The bourbon gets all its amber color during the aging process.īourbon may rest in the barrel for several years. Here, the bourbon interacts with chemicals in the barrel's wood, and about 70% of the bourbon's final flavor is determined by this step. One of these fractions is called the 'hearts,' containing mostly ethanol and water, but also small amounts of congeners, which play a big role in the final flavor of the product.Īfter distillation, the 'hearts' fraction (which is clear and resembles water) is placed in a charred oak barrel for the aging process. As the kettle heats further, chemicals with higher boiling points vaporize and then are condensed and collected.īy the end of the distillation process with a pot still, the distillate has been divided into a few fractions. So in the case of the pot still, as the kettle is heated, chemicals that have lower boiling points are collected first. The desired vapors that exit the still are condensed back to liquid form, and this product is called the distillate.ĭifferent chemical compounds have different boiling points, so distillers can separate the different chemicals by collecting the distillate at different temperatures. Pot stills result in a spirit that often contains a more complex mixture of congeners. The type of still will influence the beverages' final flavor, because pot stills often do not separate the congeners as precisely as column stills do. And different yeasts produce different amounts of congeners.Ī column still. The longer the fermentation period, the longer the yeast has to create more flavorful byproducts, which enhances the complexity of the spirit's final taste. Congeners can be esters, which produce a fruity or floral flavor, or complex alcohols, which can taste strong and aromatic. Yeast fermentation yields other byproducts besides alcohol and carbon dioxide, including flavor compounds called congeners. The fermented mash is now called 'beer.' While similar in structure and taste to the beer you might buy in a six-pack, this product still has a way to go before it reaches its final form. This process is called 'pitching the yeast.' The yeast consumes sugars and produces ethyl alcohol and carbon dioxide as byproducts during the process called fermentation-that's how the bourbon becomes alcoholic. Once distillers grind the grains from the mash bill and mix them with heated water, they add yeast to the mash. Others might use grains like rice or quinoa-but each grain chosen, and the amount of each, affects the flavor down the line. Rye makes the bourbon spicy, while wheat produces a softer, sweeter flavor.

Many distillers also use rye and wheat to flavor their bourbons. To be classified as bourbon, a spirit's mash bill must have at least 51% corn-the corn gives it that characteristic sweetness.Īlmost all bourbons also have malted barley, which lends a nutty, smoky flavor and provides enzymes that turn starches into sugars later in the production process. The mash bill refers to the recipe of grains that makes up the spirit's flavor foundation. The complex science behind this aromatic beverage reveals why there are so many distinct bourbons, despite the strict rules around its manufacture.Īll whiskeys have what's called a mash bill. I'm a bourbon researcher and chemistry professor who teaches classes on fermentation, and I'm a bourbon connoisseur myself. The other rules have more to do with the steps to make it-how much corn is in the grain mixture, the aging process and the alcohol proof. territory-although almost all is made in Kentucky. But for a whiskey to be called a bourbon, it has to adhere to very specific rules.

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